1. Field of the Invention
The present invention relates to a brake control system for a vehicle, which discharges brake fluid in a reservoir to wheel brake cylinders through a modulator by means of a hydraulic pressure pump to perform various braking controls, and especially relates to the brake control system having a function of auxiliary pressurization applied to an input of the pressure pump.
2. Description of the Related Arts
In order to reduce a force for operating a brake pedal when a vehicle is to be braked, various boosters have been employed. Among them, a vacuum booster is popular. It has a constant pressure chamber which is always communicated with an intake manifold of an engine to introduce the negative pressure therefrom, and a variable pressure chamber which is selectively communicated with the atmosphere, or communicated with the constant pressure chamber to introduce the negative pressure therefrom. Also provided is a valve mechanism, which includes a vacuum valve for communicating the constant pressure chamber with the variable pressure chamber, or blocking the communication between them, and an air valve for communicating the variable pressure chamber with the atmosphere, or blocking the communication between them. With the valve mechanism operated in response to depression of the brake pedal, the valve mechanism is actuated to open or close the vacuum valve and/or the air valve, so that a pressure difference between the constant pressure chamber and the variable pressure chamber is produced in response to the force for operating the brake pedal to increase the force, and transmit the increased force to a master cylinder.
With respect to the vacuum booster, Japanese Patent Laid-open Publication No.5-24533 discloses an electrically operated tandem brake boost apparatus, which is adapted to communicate the variable pressure chamber with the atmosphere by means of a solenoid valve, even if the brake pedal is not depressed, whereby a large pressure difference between the variable pressure chamber and the constant pressure chamber is produced to increase the output force, and which is used for automatic braking systems.
Among the automatic braking systems, an automatic pressurization is performed in various braking control systems having braking force control functions, such as a steering control by braking, which is adapted to restrain an excessive oversteer when it is determined that the excessive oversteer occurs during cornering, for example, with a hydraulic braking pressure discharged from a hydraulic pressure pump applied to a wheel cylinder operatively mounted on a front wheel located on the outside of the curve in the vehicle's path, irrespective of operation of the brake pedal. That is, the hydraulic braking pressure is automatically applied to the wheel brake cylinders by the output pressure of the hydraulic pressure pump, irrespective of operation of the brake pedal. When the braking control performed by the automatic pressurization is initiated, the hydraulic pressure pump starts to be driven. Under a condition at a low temperature, brake fluid is hardly flown in a passage, so that it will be difficult to provide a desired pressure increasing gradient. Therefore, it has been desired to supply pressurized brake fluid into an input of the pressure pump thereby to assist the automatic pressurization.
With respect to the braking control performed by the automatic pressurization, it may be appropriate to utilize the vacuum booster. In Japanese Patent Laid-open Publication No.7-81540 for example, it is assumed that whole output of the vacuum booster is used for the auxiliary pressurization. Although it is almost impossible to understand the context of that publication due to its insufficient description, it is presumed that when the auxiliary pressurization is being performed for the braking control by the automatic pressurization with respect to one hydraulic braking pressure circuit, a hold mode is selected for the other hydraulic braking pressure circuit so as not to apply the hydraulic braking pressure to the wheel cylinder.
Basically, when the braking control by the automatic pressurization is being performed with respect to one hydraulic braking pressure circuit, it is necessary to place the other hydraulic braking pressure circuit in such a condition that the hydraulic braking pressure in the wheel brake cylinder can be increased, so that the braking force could be applied to the wheel in the other hydraulic braking pressure circuit, if the brake pedal is depressed. However, in the case where the output of the vacuum booster is large, when the auxiliary pressurization is performed with respect to the one hydraulic braking pressure circuit, large braking force will be applied to the wheel in the other hydraulic braking pressure circuit. This is resulted from the fact that a power source of the vacuum booster is the negative pressure produced in the intake manifold of the engine. Therefore, the large braking force is necessarily applied to the wheel in the other hydraulic braking pressure circuit, when the negative pressure produced in the intake manifold is large. As a result, the wheel is likely locked, on a road with a relatively low coefficient of friction. In the aforementioned Japanese Patent Laid-open Publication No.7-81540, therefore, it is assumed that the hold mode is to be selected for the other hydraulic braking pressure circuit, when the brake pedal is not depressed.
However, it is inappropriate to select the hold mode for the other hydraulic braking pressure circuit, because it will be impossible to apply the braking force in response to an amount depressed by the brake pedal, with respect to the other hydraulic braking pressure circuit, when the brake pedal is depressed. As a countermeasure to this, it is desirable that when the auxiliary pressurization is performed with respect to the one hydraulic braking pressure circuit, the hold mode is to be selected for the other hydraulic braking pressure circuit, and that when the brake pedal is depressed, the pressure increase mode is to be selected.